Four novels packed into one omnibus Cities in Flight essentially tells the story of the discovery of anti-gravity and how whole earth cities are levitated into space. The various cities float around visiting other planets and doing odd jobs in return for resources.
The premise seems to be more of a 'what if?' than logical fantasy. For example, how did the engineers manage to dig under the cities to release them from the earth? James Blish is saying IF we could discover a way to live much longer lives and IF we could take earth cities into space then what would happen.
The only other negative I can think of is that the story is a little too long and one can feel like one is continuing to eat after one is satisfied. This is easily fixed - read one novel at a time. I suppose it could also be argued that this isn't exactly a page turning thriller. For pure enjoyment I would recommend other works on the list. But this is a wonderful, wonderful piece of SF.
The atmosphere is very retro, almost steampunk-esque. The storyline is very diverse and novel #1 is all based on the earth and near jupiter where the ant-grav effect is first observed. The joy of the story is in seeing the city being lodged in a very tricky situation and then having to find a way out of trouble. This makes it feel almost Star Trek like in places. But the fact that we are dealing with a city mayor and not a captain and the longevity of the characters and the presence of the city fathers adds so much more.